There are many strange and wonderful planets outside the solar system, but a newly discovered world is a true cosmic wonder. The exoplanet TOI-5205b is a gas giant about the same size and mass as Jupiter, orbiting the red dwarf star TOI-5205. But there is nothing unusual about it; Planets always revolve around the stars. TOI-5205’s size is unusual compared to a Jupiter-sized exoplanet. The star is not four times the radius of its planet, but just under 40 percent of the Sun’s radius and mass. Additionally, TOI-5205b’s orbit is disturbingly close, orbiting the red dwarf every 1.6 days.

This is the first time that such a large exoplanet has been found orbiting such a small red dwarf, and astronomers have not figured out how such a strange pair formed. This contradicts our current understanding of planet formation.

“The host star TOI-5205 is about four times the size of Jupiter,” says astronomer Shubham Kanodia of the Carnegie Institute for Science, “but somehow it managed to form a Jupiter-sized planet, which is pretty surprising.”

Red dwarfs are the smallest main sequence stars we know of; less and less and we’re starting to come back to the land of brown dwarfs, not exactly stars. They are low in mass, luminosity, and temperature, and they deplete their hydrogen stores so slowly that their estimated lifespan can reach trillions of years; this could be much longer than the current age of the universe, 13.8 billion years.

Red dwarfs are impossible to see with the naked eye because they are so small and faint; but they are the biggest stars in the Milky Way. However, of the 5,250 confirmed exoplanets at the time of this writing, only 240 are orbiting red dwarfs; only a dozen of them are Jupiter-sized or larger.

What we understand about planet formation suggests that such star-exoplanet pairings are unlikely. Stars are formed by clusters in dense clouds of gas and dust. As they grow, the material around them coalesces into a disc that falls into the baby star, similar to water swirling in a sewer. When a star is large enough, it releases powerful winds that blow away nearby material, stopping the star’s growth.

What remains in this disk form objects that orbit the star, coalesce, and gradually turn into planets. To create a gas giant, our current simulations suggest that about 10 Earth masses of rock disc material is required to form a planetary core, which then accumulates gas to form a giant expanded atmosphere. This process must also occur relatively quickly before expelling the remnants of the small star disk. Our models also suggest that there must be not enough material in the disk of a small red dwarf star for this process to occur on the required time scale.